Electroluminescence display device

ABSTRACT

In an organic EL display device including a TFT ( 40 ) and an organic EL element ( 60 ) formed on an insulating substrate ( 10 ) of a glass substrate, insulating films, namely, a planarization insulating film ( 17 ), an interlayer insulating film ( 15 ), and a gate insulating film ( 12 ), for forming the TFT ( 40 ) are not provided at a region for forming the organic EL element ( 60 ). Instead, an opening ( 65 ) formed by removing these insulating films is provided to expose the glass substrate ( 10 ), and at this exposed region an anode ( 61 ), an emissive element layer ( 62 ), and a cathode ( 63 ) are formed in this order. Consequently, light emitted from the emissive element layer ( 62 ) is prevented from reflecting at interfaces between these insulating films ( 12, 15 , and  17 ), and efficiently exits from the glass substrate ( 10 ) side.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. Pat. No.09/521,747, filed on Mar. 9, 2000, now U. S. Pat. No. 6,492,778 which isherein incorpoiated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electroluminescence (EL) displaydevice comprising a thin film transistor (TFT) and an EL element inwhich the TFT is employed as a switching element.

2. Description of the Related Art

FIG. 1 is a plan view showing a display pixel portion of an EL displaydevice of a related art, and FIGS. 2A and 2B are cross sectional viewsillustrating the EL display device taken along the lines A—A and B—B inFIG. 1, respectively.

Referring to FIG. 1, a display pixel is formed in a region surrounded bygate signal lines 51 and drain signal lines 52. A first TFT 30 isprovided near an intersection of these signal lines, and has a source 13s serving as a capacitor electrode 55 which forms a capacitor with astorage capacitor electrode line 54, and connected to a gate 41 of thesecond TFT 40. The second TFT 40 has a source 43 s connected to an anode61 of an organic EL element 60, and a drain 43 d connected to a powersource line 53 for driving the organic EL element.

The storage capacitor electrode line 54 is disposed near the TFTsinparallel to the gate signal line 51. The line 54 is composed ofchromium or the like and stores electric charges to form a capacitorwith the capacitor electrode 55 connected to the source 13 s of the TFTwith a gate insulating film 12 interposed therebetween. The storagecapacitor is provided to retain a voltage applied to the gate electrode41 of the second TFT 40.

Thus, display pixels each having the organic EL element 60 and the TFTs30 and 40 are arranged in a matrix on a substrate 10, thereby forming anorganic EL display device.

Referring to FIGS. 2A and 2B, the organic EL display device is composedof the TFTs and the organic EL element formed in succession on asubstrate 10, such as a substrate formed of glass, synthetic resin, orthe like, an electrically conductive substrate, and a semiconductorsubstrate. When a conductive or semiconductor substrate is used for thesubstrate 10, an insulating film of SiO₂, SiN, or the like is firstformed on the substrate 10 before the TFTs 30 and 40 and the organic ELdisplay element 60 are provided.

The first TFT 30 for switching operation will next be described.

As shown in FIG. 2A, on the insulating substrate 10 of quartz glass,non-alkaline glass, or the like, the gate signal line 51 formed ofrefractory metal, such as chromium (Cr) and molybdenum (Mo), and servingas a gate electrode 11, and the drain signal line 52 formed of aluminum(Al) are provided, and the power source line 53 formed of Al and servingas a driving power source for the organic EL element is disposed.

Thereafter, a gate insulating film 12 and an active layer 13 of apolysilicon (p-Si) film are formed in this order. The active layer 13 isof a so-called LDD (lightly doped drain) structure, and the source 13 sand a drain 13 d are provided on the outer sides thereof.

An interlayer insulating film 15 composed of an SiO₂ film, an SiN film,and an SiO₂ film formed in this order is provided over the entiresurface, covering the gate insulating film 12, the active layer 13, anda stopper insulating film 14. A drain electrode 16 is formed by fillinga metal such as Al in a contact hole provided corresponding to the drain13 d. Further, a planarization insulating film 17 is formed of organicresin or the like over the entire surface for planarization.

The second TFT 40 for driving the organic EL element 60 will next bedescribed.

As shown in FIG. 2B, the gate electrode 41 is formed of refractorymetal, such as Cr and Mo, on the insulating substrate 10 formed ofquartz glass, non-alkaline glass, or the like.

The gate insulating film 12 and an active layer 43 formed of a p-Si filmare provided in succession.

The active layer 43 includes an intrinsic, or substantially intrinsic,channel 43 c provided over the gate electrode 41, and the source 43 sand the drain 43 d provided on respective sides of the channel 43 c byion doping.

The interlayer insulating film 15 composed of an SiO₂ film, an SiN film,and an SiO₂ film formed in this order is next provided over the entiresurface to cover the gate insulating film 12 and the active layer 43.The power source line 53 connected to a driving power source 50 isformed by filling a metal such as Al in a contact hole providedcorresponding to the drain 43 d. The planarization insulating film 17 oforganic resin or the like is formed over the entire surface for thepurpose of planarization. A contact hole is formed in the planarizationinsulating film 17 and the interlayer insulating film 15 at a positioncorresponding to the source 43 s. A transparent electrode formed of ITO,i.e. the anode 61 of the organic EL element, is formed on theplanarization insulating film 17 so as to contact the source 13 sthrough the contact hole.

The organic EL element 60 includes the anode 61 formed of a transparentelectrode of ITO or the like, an emissive element layer 62 of an organiccompound, and a cathode 63 of a magnesium-indium alloy, formed in thisorder. The cathode 63 is provided over the entire surface of thesubstrate 10 which forms the organic EL display device, i.e. over theentire plane of the FIG. 1.

In the organic EL element, holes and electrons injected from the anodeand cathode, respectively, are recombined in the emissive layer toexcite organic molecules forming the emissive layer, thereby producingexcitons. Light is released from the emissive layer during the processin which the excitons deactivate, and this release of light from thetransparent anode through the transparent insulating substrate resultsin the emission of light.

With such a configuration, after the anode 61 is formed, an insulatingfilm 64 is formed in the peripheral region of the anode 61 (the regionexcluding the area surrounded by broken lines) to prevent a shortcircuit between the cathode 63 and the anode 61, which would otherwisebe generated from a crack in the emissive layer resulting from adifference in level created by the thickness of the anode 61. Theemissive element layer 62 and the cathode 63 are next formed. Lightemitted from the emissive element layer 62 exits after being transmittedthrough the insulating substrate 10.

It should be noted that while light emitted from the emissive elementlayer 62 advances radially outward from the emissive layer 62, not allcomponents of the light emitted from the emissive layer 62 toward thesubstrate 10 reach the insulating substrate 10, and some light isreflected by, for example, the surface of the planarization insulatingfilm, or attenuated in the film.

This results from a difference in materials between the interlayerinsulating film and the planarization insulating film, especially from adifference in refractive index between these insulating films. The lightmay be partly reflected at the interface between respective insulatingfilms, or even if transmitted through the film, the light may beattenuated as it is repeatedly reflected within the insulating filmdepending on the angle incident to the insulating film.

FIG. 3 shows how light radiated from the emissive layer travels towardthe substrate.

Referring to FIG. 3, the emitted light incident on the SiN film advancesat an angle α1 from a normal line C to the SiN film, and is thenreflected at the interface between the SiN film (having a refractiveindex n1=2.0) and the SiO₂ film (having a refractive index n2=1.46).Assuming that the reflected ray forms an angle α2 with the normal lineC, an equation n1×Sin α1=n2×Sin α2 holds true from Snell's law, andα1=Sin⁻¹ (n1/n2)≈47°. As the value n1 is larger than the value n2 inthis example, light is totally reflected at this interface when theangle α1 exceeds 47°.

Therefore, only the components with the angle α1 smaller than 47° cantravel downward, i.e. to the SiO₂ film, and efficiency of extractinglight at this interface is 47°/90°≈52%.

As the lower region of the organic EL element includes films havingdifferent refractive indices, there is always an angle that causes totalreflection when light travels from a substance with a larger refractiveindex to a substance with a smaller refractive index. As a result, lightprojected from the lower glass substrate will always have a reducedintensity.

Thus, light radiated from the emissive layer is diminished by the timeit exits the insulating substrate 10, resulting in an inefficient use ofthe light and darkened display of the EL display device.

SUMMARY OF THE INVENTION

The present invention was conceived in view of the above-describedproblems, and aims to provide an EL display device allowing lightemitted from an emissive layer to efficiently exit from an insulatingsubstrate, to thereby obtain a bright display.

According to one aspect, the present invention provides a light emissivedisplay device comprising, for each pixel, an emissive element composedof a first electrode of a transparent, electrically conductive material;an emissive element layer; and a second electrode of an opaque,electrically conductive material formed in this order, and a switchingelement for driving the emissive element. At least part of the firstelectrode of the emissive element is formed in a direct contact with asurface of the transparent substrate.

According to another aspect of the present invention, the switchingelement is formed on the transparent substrate, and an insulating layeris formed over the switching element to insulate the emissive elementfrom the switching element except for a necessary region and separatethese two elements. The insulating layer includes an opening in eachpixel region, and the first electrode of the emissive element covers thesurface of the transparent substrate exposed by the opening.

According to a still another aspect of the present invention, theswitching element is a thin film transistor, and a gate insulating layeris formed on the transparent substrate for insulating a gate of the thinfilm transistor from an active layer, and the opening of said insulatinglayer passes through said gate insulating.

According to a further aspect of the present invention, the insulatinglayer for insulating a region of the emissive element from the switchingelement and for separating the two elements includes an interlayerinsulating layer and a planarization insulating layer for planarizing asurface of a region located over the switching element.

For example, the interlayer insulating layer may be composed of multiplelayers including silicon dioxide and/or silicon nitride, and theplanarization insulating layer includes insulating resin. The firstelectrode may include indium tin oxide. The gate insulating layer mayinclude, for example, silicon dioxide The first electrode may include,for example, indium tin oxide.

In the device structured as described above, no layers with differentrefractive indices exist in a region where the first electrode directlycontacts the transparent substrate, so that undesirable reflection oflight emitted from the emissive element layer at an interface betweenlayers with different refractive indices can be prevented, therebymaking it possible for light to efficiently pass outside after beingtransmitted through the first electrode and the transparent substrate.

According to a further aspect of the present invention, the switchingelement is formed on the transparent substrate, an insulating layer isformed over the switching element for insulating the emissive elementfrom the switching element except for a necessary region and separatingthe two elements, and includes an opening, in each pixel region, havinga diameter increasing from the center of the region toward an outer end.

According to a further aspect of the present invention, the switchingelement is formed on the transparent substrate, and an insulating layeris formed over the switching element for insulating the emissive elementfrom the switching element except for a necessary region and separatingthe two elements, and has an opening, in each pixel region, having adiameter increasing from the center of the region toward an outer end.Further, the first electrode is formed as an individual electrode ineach pixel, and a planarization insulating film is formed in thevicinity of an edge of the first electrode for separating the firstelectrode from the emissive element layer and the second electrode andhaving a tapered surface with a thickness increasing from the center ofthe first electrode toward an outer end.

Tapering the opening in the insulating film for insulating andseparating the emissive element from the switching element preventsgeneration of a discontinuity which would otherwise be generated by adifference in level created in the vicinity of an end of the opening inthe first electrode, the emissive element layer, the second electrode,and the like formed to cover the opening.

A planarization insulating film is formed in the vicinity of the end ofthe first electrode for separating the first electrode from the emissiveelement layer and the second electrode, and having a tapered surfacewith a thickness increasing from the center of the first electrodetoward an outer end. This will lead to prevention of a discontinuity inthe second electrode in the vicinity of the end of the first electrode.

According to a further aspect of the present invention, the emissiveelement is an electroluminescence element.

According to a further aspect of the present invention, the emissiveelement is an organic electroluminescence element with the emissiveelement layer formed of an organic compound.

Forming the emissive element layer of an organic compound is extremelyadvantageous in display devices and the like because this configurationoffers a wide variety of displayable colors and a wide range of optionsfor the material used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing an EL display device of a related art.

FIGS. 2A and 2B are cross sectional views taken along the lines A—A andB—B in FIG. 1, respectively.

FIG. 3 illustrates the path of light traveling through the emissiveelement layer.

FIG. 4 is a plan view showing an EL display device according to anembodiment of the present invention.

FIG. 5 is a cross sectional view taken along the line B—B in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An EL display device according to the present invention will next bedescribed.

FIG. 4 is a plan view illustrating a display pixel portion of an ELdisplay device of the present invention, and FIG. 5 is a cross sectionalview taken along the line B—B in FIG. 4. The cross section taken alongthe line A—A in FIG. 4 is not illustrated because it is the same as thatshown in FIG. 2A referred to in the above description.

Referring to FIG. 4, a display pixel including an organic EL element isformed in a region surrounded by gate signal lines 51 and drain signallines 52. A first TFT 30 is provided near an intersection of thesesignal lines, and has a source 13 s serving as a capacitor electrode 55which forms a capacitor with a storage capacitor electrode line 54, andconnected to a gate 41 of a second TFT 40. The second TFT 40 has asource 43 s connected to an anode 61 of an organic EL element 60, and adrain 43 d connected to a power source line 53 for driving the organicEL element.

The storage capacitor electrode line 54 is disposed near the displaypixel in parallel to the gate signal line 51. The line 54 is made ofchromium or the like, and stores electric charges to form a capacitorwith the capacitor electrode 55 connected to the source 13 s of the TFTwith a gate insulating film 12 interposed therebetween. The storagecapacitor is provided to retain a voltage applied to the gate electrode41 of the second TFT 40.

Thus, display pixels each having the organic EL element 60, and the TFTs30 and 40 are arranged in a matrix on a substrate 10, to thereby form anorganic EL display device.

Referring to FIG. 5, the organic EL display device is composed of theTFTs and the organic EL element formed in succession on a substrate 10.The substrate 10 may be a substrate formed of glass, synthetic resin, orthe like, or a conductive or semiconductor substrate. When a conductiveor semiconductor substrate is used for the substrate 10, an insulatingfilm of SiO₂, SiN, or the like is first formed on the substrate 10before the TFTS and the organic EL display element are provided.

In the present embodiment, both the first and second TFTs 30 and 40 areof a so-called bottom gate type, in which gate electrodes 11 and 41 areprovided under active layers 13 and 43, respectively, and a p-Si film isused for the active layer which is a semiconductor film. Further, inthis embodiment, the gate electrodes 11 and 41 are of the double gatestructure.

Since the first TFT 30 for switching operation has the same structure asthat described with reference to FIG. 2A, description thereof will notbe repeated.

The second TFT 40 for driving the organic EL element 60 will next bedescribed with reference to FIG. 5.

As shown in FIGS. 4 and 5, the gate electrode 41 of refractory metal,such as Cr and Mo, is formed on the insulating substrate 10 of quartzglass, non-alkaline glass, or the like. A gate insulating film 12 isformed of SiO₂ or the like to cover the gate electrode 41, and theactive layer 43 of a p-Si film is formed on the gate insulating film 12in a region for forming the second TFT 40.

The active layer 43 includes an intrinsic, or substantially intrinsic,channel 43 c provided over the gate electrode 41, and the source 43 sand the drain 43 d provided on respective sides of the channel 43 c byion doping.

A multi-layered interlayer insulating film 15 composed of, for example,an SiO₂ film, an SiN film, and an SiO₂ film in this order is next formedover the entire surface to cover the gate insulating film 12 and theactive layer 43. The power source line 53 connected to a driving powersource is formed by filling a metal such as Al in a contact holeprovided in the layer 15 at a position corresponding to the drain 43 d.A planarization insulating film 17 of organic resin or the like isformed over the entire surface for the purpose of planarization. Acontact hole is formed in the planarization insulating film 17 and theinterlayer insulating film 15 at a position corresponding to the source43 s. A transparent electrode formed of ITO , i.e. the anode 61 of theorganic EL element, is formed on the planarization insulating film 17 soas to contact the source 43 s through the contact hole.

Thus, part of the organic EL element 60 is disposed over the TFT 40 withthe insulating films (15, 17) interposed therebetween. The TFT 40 andthe element 60 are insulated from each other by the insulating films(15, 17) except for a necessary region, that is, part of the anode ofthe EL element and the source 43 s of the TFT connected through thecontact hole. The insulating films (15, 17) horizontally separate theTFT and the EL element from each other.

The organic EL element 60 is composed of the anode 61 formed of atransparent electrode of ITO or the like, an emissive element layer 62,and a cathode 63 of a magnesium-indium alloy, formed in this order. Thecathode 63 is provided over the entire surface of the substrate 10 usedfor forming the organic EL display device shown in FIG. 4, i.e. over theentire plane of the sheet. The emissive element layer 62 is composed of,for example, a first hole-transport layer of MTDATA(4,4′,4″-tris(3-methlphenylphenylamino)triphenylamine), a secondhole-transport layer of TPD(N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), anemissive layer of Bebq2 (bis(10-hydroxybenzo[h]quinolinato)beryllium)including quinacridone derivatives, and an electron-transport layer ofBebq2, formed in succession on the anode 61. As shown in FIG. 5, whilethe emissive layer is formed as a separate pattern for each pixel as isthe anode 61 and occupying a little wider area than the anode 61, thefirst and second hole-transport layers and the electron-transport layersandwiching the emissive layer are formed over the entire surface of thesubstrate and shared by respective pixels similarly to the cathode 63.

In the organic EL element, holes and electrons injected from the anodeand cathode, respectively, are recombined in the emissive layer toexcite organic molecules forming the emissive layer, thereby producingexcitons. Light is released from the emissive layer during the processin which the excitons deactivate, and this release of light from thetransparent anode through the transparent insulating substrate causeslight to be emitted to the outside.

The gate insulating film 12, the interlayer insulating film 15, and theplanarization insulating film 17 formed for providing the TFT 40 areremoved at a region for forming an organic EL display element beforeformation of this element.

In other words, an opening 65 is formed in the insulating films 12, 15,and 17 at the region where the organic EL element is to be provided, andtherefore the surface of the insulating substrate 10 is exposed beforethe organic EL element is formed.

Accordingly, the anode 61 is formed directly on the exposed region ofthe insulating substrate 10 and on part of the portion located over theTFT 40. Thus, as no insulating films with different refractive indicesexist on the light emitting side of the EL element, total reflection ofthe light due to differing refractive indexes does not occur. As aresult, light can be more efficiently extracted from the insulatingsubstrate 10 side, to thereby achieve a bright display.

Removal of the insulating films provided when the TFT 40 is formed fromthe region for forming the organic EL element may be performed each timean insulating film is formed. Alternatively, the insulating films may beremoved from the region for forming the organic EL element at the sametime the contact hole is formed in the planarization insulating film 17and the interlayer insulating film 15 at the position corresponding tothe source 43 s after formation of the TFT 40 is completed. The latteris preferable because it does not increase the number of manufacturingsteps. In addition, the opening 65 resulting from removal of the,insulating films preferably has a diameter outwardly increasing from thecenter thereof, as shown in FIG. 5. Such a tapered opening 65 preventsdiscontinuity in the anode, the emissive layer, and the cathode at anend of the opening, and prevents short circuiting between the anode andthe cathode.

On the exposed portion of the insulating substrate 10, ITO, a materialof the anode 61, is deposited and then etched. Outside of the regionindicated by the broken lines, a planarization insulating film 19 isnext formed around the thus formed anode 61. This film is provided forpreventing short circuiting between the cathode 63 and the anode 61which might otherwise be caused by a crack in the emissive layerresulting from a difference in level generated by the thickness of theanode 61. The layers of the above-described materials are deposited onthe anode 61 to form the emissive layer 62. On this layer 62, thecathode 63 shared by respective organic EL elements is formed of anopaque material, namely, a magnesium-indium alloy, over the entiresurface of the substrate 10 including the region located over the TFT40.

In the thus fabricated organic EL device, as the organic EL element isformed in the opening 65, i.e. directly on the insulating substrate 10,no components of light emitted from the emissive layer are reflection bythe insulating films forming the TFT 40, namely, the planarizationinsulating film 17, the interlayer insulating film 15, and the gateinsulating film 12. As a result, a brighter organic EL display devicecan be obtained. Because emitted light is more efficiently utilized,this brighter display can be achieved without increasing the currentsupplied to the organic EL element, which contributes to increasing thelifespan of the element.

What is claimed is:
 1. A light emissive display device, comprising, foreach pixel: an emissive element composed of a first electrode of atransparent, electrically conductive material; an emissive elementlayer; and a second electrode of an opaque, electrically conductivematerial, formed in this order; and a switching element for driving saidemissive element; wherein at least part of said first electrode of saidemissive element is formed in direct contact with a surface of atransparent substrate.
 2. The device according to claim 1, wherein saidfirst electrode includes indium tin oxide.
 3. A light emissive displaydevice, comprising, for each pixel: an emissive element composed of afirst electrode of a transparent, electrically conductive material; anemissive element layer; and a second electrode of an opaque,electrically conductive material, formed in this order; and a switchingelement for driving said emissive element; wherein at least part of saidfirst electrode of said emissive element is formed in direct contactwith a surface of a transparent substrate, and wherein said firstelectrode includes indium tin oxide.
 4. The device according to claim 3,wherein said emissive element is an electroluminescence element.
 5. Thedevice according to claim 3, wherein said emissive element is an organicelectroluminescence element including said emissive element layer of anorganic compound.
 6. The device according to claim 3, wherein lightemitted from said emissive element passes through said first electrodeand said transparent substrate.
 7. A light emissive display device,comprising, for each pixel: an emissive element composed of a firstelectrode of a transparent, electrically conductive material; artemissive element layer; and a second electrode of an opaque,electrically conductive material, formed in this order; and a switchingelement for driving said emissive element; wherein said first electrodeof said emissive element is an individual electrode separately formedfor each pixel, at least part of said first electrode being formed indirect contact with a surface of a transparent substrate.
 8. The deviceaccording to claim 7, wherein said first electrode includes indium tinoxide.